Blisters indicate pressure, especially if they have raised rims. The holes are only 1/32 or so, but 200x makes them look huge. However, it is enough to cause weeping. Bubbles are normal for off gassing spars and can be remedied. Blisters, even if a few typically indicate the body has been compromised. Bubbles can come from a glaze, and sometimes blisters: but blisters more often come from the clay; sulfides and spars.